Normally, devices for the plasma treatment of substrates by means of high frequency contain two electrodes, of which one is formed by the vacuum chamber and/or the substrate holder and both comprise metallic material. It is well known in this connection that a voltage, which is predominantly negative relative to the plasma, is formed at that electrode, which has an effective surface area that is smaller than the surfaces acting as the counter electrode. The electrode, biased negatively, is therefore also regularly referred to here as the "cathode".
When using a plate-shaped substrate holder, which is at the same potential as the vacuum chamber and thus forms the one electrode, on the one hand, and using a plate-shaped electrode lying opposite the substrate holder on the other, a negative potential is developed at the lastmentioned electrode because of the area ratio, which is necessarily fixed in such an arrangement. As a result, material (=target) on this electrode (=cathode) is atomized and deposited on the substrate. If such an arrangement is to be used for etching, target and substrate have to be exchanged correspondingly. When a d.c. voltage is used, the polarity of the electrodes is necessarily given by their connection to the respective pole of the d.c. voltage source. From the German Offenlegungsschrift No. 2,151,590, for example, the use of a hollow cathode with an edge that protrudes in the direction of the substrate carrier in order to improve the uniformity of the distribution of layer thickness, is known. However, when a d.c. voltage is used, this protruding edge does not lead to a reversal of polarity.
From the German Patent No. 2,241,229, a device corresponding in general, to the device described supra is known. In this device, the surface area of the hollow electrode, because of its edge, is larger the surface of the other electrode (the substrate holder) directly facing the hollow space. As a result, when a high frequency is used, the hollow electrode assumes the function of an anode, so that, so to say, there is a reversal of the relationships with respect to the bias voltage. In a simplified way, this can be explained by considering the edge of the hollow anode to form the boundary of the discharge space, relative to which the metallic parts of the device, lying outside of this boundary, no longer exert the function of an electrode, so that the relationships are determined exclusively by the parts of the surface of the hollow anode on the one hand and of the substrate holder on the other, which face one another. This can be called an "edge effect".
However, for certain devices employing a continuous transport of substrate and using this principle, the known solution leads to some difficulties. For instance, if the substrate holder with the substrate is removed, at least the metallic baseplate of the vacuum chamber, which usually has a larger surface than the hollow electrode and is connected to ground, acts as a counter electrode. In this case, in a complete reversal of the potential relationships, the negative voltage develops on the hollow electrode, which now functions as a cathode.
It follows from this that the size of the gap between the edge of the hollow electrode on the one hand and the substrate or substrate holder on the other, is of essential importance for the development of the potential distribution in the region of the space filled by the plasma. Since said gap represents the connection between the plasma and its surroundings, the size of the gap determines whether the hollow electrode functions as the anode or the cathode of the arrangement. It was established by experiment that an enlargement of said gap by an amount of about 1 mm, drastically increases the influence of the remaining metal parts of the device and moreover with the result that a negative bias voltage is formed on the hollow electrode instead of on the substrate. With continuous devices, in which the substrate is disposed on movable substrate holders, such enlargements of the gap are practically unavoidable, especially when such substrate holders are moved at a distance from one another past the opening of the hollow electrode. The gap in question can also not be kept arbitrarily small, not only because this would make it necessary to guide the substrate holder with exceptional precision, but also because the substrate, lying on the surface of the substrate holder, would change the gap width during its passage through the installation, unless one takes the very expensive route of using a different substrate holder for each shape and size of substrate, a substrate holder in which the substrate is disposed recessed in and level with the surface of the substrate holder. With this, the range of applications of the known principle is greatly limited.
It is therefore an object of the invention to improve a device of the initially described kind so that it is insensitive to an enlargement of the gap and can be used especially also in devices with a continuous or quasi continuous transport of substrate, that is, in so-called "in-line installations".